1. Field of the Invention
The present invention relates to an input device which inputs information according to proximity of an object, and for example, to an input device, such as a touch pad or a touch sensor, including detection electrodes whose electrostatic capacitance changes according to proximity of an object.
2. Description of the Related Art
As a user interface device of an information apparatus, such as a smartphone, devices, such as a touch pad and a touch panel, including a sensor configured to detect a touch position of a finger or a pen become widespread. While there are various types, such as a resistive film type and an optical type, for a sensor configured to detect a touch position of an object, in recent years, an electrostatic capacitance type sensor is mounted in many mobile apparatuses since a configuration is simple and reduction in size is achieved.
The electrostatic capacitance type sensor primarily includes a mutual capacitance type sensor which detects change in electrostatic capacitance between a drive electrode and a detection electrode, and a self-capacitance type sensor which detects change in electrostatic capacitance of a detection electrode with respect to the ground. The mutual capacitance type sensor is capable of detecting change in electrostatic capacitance at a plurality of positions on the detection electrode and is thus suitable for multi-point detection compared to the self-capacitance type sensor. The self-capacitance type sensor directly detects change in electrostatic capacitance between a proximate object and the detection electrode, and thus, there is an advantage in that detection sensitivity is high compared to the mutual capacitance type sensor. For this reason, in a case of implementing a hovering function of detecting an operation of a finger at a position separated from an operation surface, or the like, the self-capacitance type sensor having high sensitivity is advantageous.
FIGS. 14A and 14B are diagrams showing the configuration of a general self-capacitance type sensor. FIG. 14A shows the configuration of a circuit, and FIG. 14B shows the waveforms of an AC voltage Vs and an output voltage Vout. If an object (finger or the like) regarded as a ground potential or a fixed potential is proximate to a detection electrode 81, a capacitor Cs is formed between the detection electrode 81 and the ground. The capacitor Cs changes according to the distance between the object and the detection electrode 81. For this reason, the capacitance value of the capacitor Cs is measured, thereby obtaining information relating to the proximity state of the object.
Since an output terminal of an operational amplifier 84 is connected to an inverting input terminal through a capacitor Cf, the voltage of the inverting input terminal substantially becomes equal to that of a non-inverting input terminal according to a negative feedback operation of the operational amplifier 84. That is, the voltage of the detection electrode 81 connected to the inverting input terminal of the operational amplifier 84 substantially becomes equal to the AC voltage Vs applied to the non-inverting input terminal. With this, a voltage which is substantially becomes to the AC voltage Vs is generated in the capacitor Cs, and an AC current according to the AC voltage flows in the capacitor Cs. An AC current which is substantially equal to that of the capacitor Cs flows in the capacitor Cf. Accordingly, the amplitude of the AC voltage generated in the capacitor Cf is proportional to the capacitance value of the capacitor Cs. The output voltage Vout becomes a voltage obtained by adding the voltage generated in the capacitor Cf and the AC voltage Vs. As shown in FIG. 14B, the output voltage Vout has amplitude greater than the AC voltage Vs.
Even in the self-capacitance type sensor having comparatively high sensitivity, since change in electrostatic capacitance due to the proximity of the finger is extremely minute, it is desirable to increase sensitivity as much as possible in preventing the influence of noise. In order to increase sensitivity in the circuit shown in FIG. 14A, the current flowing in the capacitor Cs has to be increased by increasing the amplitude of the AC voltage Vs. However, as shown in FIG. 14B, since the output voltage Vout has amplitude larger than the AC voltage Vs, if the amplitude of the output voltage Vout increases significantly, the output voltage Vout exceeds the power supply voltage range (VDD to GND). Actually, as shown in FIG. 14B, since the output voltage Vout does not exceed the power supply voltage range (VDD to GND), the output voltage Vout is restricted within the power supply voltage range and has a distorted waveform. The same problem occurs even if the AC voltage Vs is a sine wave or has other waveforms (square wave or the like), and remains unsettled even if the capacitor Cf is replaced with a resistive element. Accordingly, in the circuit shown in FIG. 14A, it is not possible to significantly increase the amplitude of the AC voltage Vs for driving the detection electrode 81, and there is a problem in that it is difficult to increase sensitivity.
In the detection electrode 81, in addition to the capacitor Cf, there is a parasitic capacitor having no relation with the proximate object. As a method of reducing a parasitic capacitor between the detection electrode 81 and a surrounding conductive substance, for example, a method which shields the detection electrode 81 with an electrode pattern having the same potential as the AC voltage Vs is considered. However, as shown in FIG. 15, there is a parasitic capacitor Cp in the input terminal of an operational amplifier 84. The parasitic capacitor Cp is caused by an element structure of a transistor 85, and cannot be removed with a shield or the like. If there is such a parasitic capacitor, apparently, the capacitance value of the capacitor Cs increases, and the AC voltage of the capacitor Cf increases. For this reason, the amplitude of the AC voltage Vs should further decrease, and there is a problem in that sensitivity decreases. Usually, while an error component of the parasitic capacitor is subtracted from the detection result of electrostatic capacitance using an analog or digital method, such processing in the post stage has no effect of decreasing the output voltage Vout of the initial stage circuit shown in FIG. 14A. For this reason, the decrease in sensitivity described above occurs inevitably.
Recently, when there is a demand for low power consumption for various purposes, high sensitivity is required. However, in the technique of the related art, it is difficult to achieve high sensitivity while suppressing a power supply voltage to a low level.